SPRINGFIELD - The massive explosion that rocked Springfield’s entertainment district, leveling a strip club and injuring 21 people, was caused by “human error” on the part of a Columbia Gas of Massachusetts employee, according to Massachusetts Fire Marshal Stephen D. Coan.

Officials said a block-by-block investigation since Friday’s blast indicates the city’s gas system is safe, intact and functioning properly.

On Sunday, the president of Columbia Gas of Massachusetts announced his company would be compensating the hundreds of victims of the explosion because it was a company error that damaged at least 45 buildings in downtown Springfield.

The investigation will now focus on the Columbia Gas employee responsible for puncturing a gas line with a tool, and whether proper protocol was followed in the build-up to Friday’s explosion. The blast injured 21 people and prompted a large-scale evacuation of the city’s downtown entertainment district, Coan said.

“Human error¦...¦is what the cause of the explosion was,” Coan said Sunday, attributing the origins of the blast to a utility worker who punctured an underground gas line with a metal probing tool.

The incident started with a 4 p.m. report of a strong smell of natural detected in the basement of the Scores Gentlemen’s Club, said Columbia Gas President Stephen H. Bryant.

The employee, using electronic monitors, found no trace of gas in the basement. He then went outside to see if the smell could be coming from a line leading to the building, Bryant said.

“We do not know what the original smell of gas was,” Bryant said. “It is not unusual to get a call. A lot of things smell like gas.”

The standard procedure is to use a metal tool to puncture the ground near a pipe to see if there are pockets of gas in the ground. Instead the employee punctured the pipe near the foundation of the club, he said.

“I don’t think there was any question that a (Columbia Gas) employee pushed the probe through the line which released the gas that caused the explosion,” Bryant said.

Coan said the puncture caused gas to escape into the club. Gas levels inside the multistory brick building reached such a high volume that any number of possible ignition sources could have triggered the explosion, Coan said.

Street markings showing the location of underground gas lines were incorrectly placed, Coan said, adding that further investigation by the state Department of Public Utilities (DPU) will focus on why that was the case.

But Bryant disputed that the markings were wrong. He said the employee did follow standard procedure for locating the line and testing it; The problem was the line took an unexpected turn so when he inserted the probe in a spot near the foundation, he punctured the pipe.

It took about 30 minutes for Columbia Gas workers to arrive and shut off the gas.

The pipe was punctured at about 4:20 p.m., the gas was turned off about 30 minutes later and the explosion happened at about 5:25 p.m.

Bryant said the employee who punctured the line was not qualified to turn off the gas line and the company had to dispatch someone who was. It took a second employee about 25 minutes to arrive.

He praised the employee for quickly calling the company to dispatch another employee to turn off the line, phoning the Fire Department and alerting the club to start evacuating.

State Energy and Environmental Affairs Secretary Richard K. Sullivan Jr., who oversees the DPU, will oversee the investigation.

“We are transitioning the investigation to the DPU,” Coan said, speaking to reporters at a Sunday afternoon news conference at Springfield Fire Department headquarters on Worthington Street.

Officials did not publicly identify the worker who punctured the line, nor did they indicate if the worker is facing any sort of punitive action by Columbia Gas. Bryant also declined to identify the man but said he was experienced and followed proper procedures.

Sullivan said the DPU will focus on the utility company’s protocols and regulations.

“The company has fully cooperated, giving us access to all of their employees for interviews,” Sullivan said. “Now, we will be looking at issues of possible regulation violations as well as the conduct of employees before, during and after the explosion.”

Sullivan said investigators will examine whether all necessary resources were provided in a timely manner. They will also look into the question of the incorrect street markings.

“We will be conducting a thorough investigation that may not be done in days, but hopefully in weeks. We will move as quickly as possible,” Sullivan said.

Bryant said his company is also conducting an internal investigation to see if the explosion could have been prevented.

“We will put tremendous effort in to see what we can do in the future,” he said.

Twelve of the 14 Springfield firefighters who responded to the initial gas leak report were injured in the explosion, according to Springfield Fire Commissioner Joseph A. Conant.

“We’re hoping that our firefighters recover quickly and get back to work,” Conant said.

Firefighters sustained everything from knee injuries to bruises to concussions from the impact of the blast. Two firefighters suffered severe burns to their faces and heads, Conant said, adding that they will likely take longer to recover. “It could have been far worse than it was,” he said.

Others injured include Columbia Gas workers, city police officers and a photo journalist. None of the injuries were life-threatening, officials said.

Friday’s 5:25 p.m. blast leveled Scores and shut down much of the city’s entertainment district, with some sections remaining off limits until further notice. Worthington Street is expected to be closed between Chestnut and Spring streets for at least several more days, and yellow caution tape now adorns numerous damaged facades within the blast zone near the corner of Worthington and Chestnut streets.

In addition to the strip club, the explosion damaged more than 40 other structures within a multiblock radius of the blast site. Three structures, which contain about 115 housing units, have already been condemned, while other structures are still being assessed.

The number of residents who lived in the condemned buildings is unknown, said Thomas T. Walsh, spokesman for Mayor Domenic J. Sarno.

Walsh and Bryant said they have no estimate of the monetary damages from the blast as of Sunday night.

Starting Monday, Columbia Gas will staff a temporary office in Room 220 of Springfield City Hall from 10 a.m. to 5 p.m. to help residents file claims for property damage. Bryant said he did not know how many days the employees will be there, but said it will be at least two. People can also call 800-869-1876, extension 1, to talk to a representative. More information is also on the home page of the company website columbiagasma.com.

Bryant said the priority will be to assist the families who have to be moved.

Sarno said Sunday that Columbia Gas officials have been “extremely cooperative,” making themselves available at every step of the investigation. Utility officials also have vouched for the safety of the gas system after they conducted a block-by-block assessment of the infrastructure, the mayor said,

“They have taken responsibility,” Sarno said.

This story was written by staff writers Conor Berry, Elizabeth Roman and Jeanette DeForge